Materials such as lithium niobate and lithium tantalate utilize the electro-optic properties of the crystal material for sensing electric field. The electric field modulates the optical beam propagating through the crystal.
In an electro-optic crystal, the refractive index of the material changes in proportion to the amplitude and direction of the applied electric field. The resulting change in the index of refraction of the crystal leads to a phase modulation in the light propagating in the crystal. The phase modulation of the light as it emerges at the output of the crystal is a direct measure of the electric field applied to the crystal.
Optical detectors measure the amplitude or intensity of the light beam but cannot measure the phase information in the light beam. One method of measuring phase modulation consists of converting the resultant phase shift to an amplitude or intensity modulation through the use of optical interferometers.
Optical interferometers can be fabricated using a bulk crystal material or an integrated optical waveguide in a crystal substrate. The integrated optical version is more efficient and has several advantages over the bulk crystal device.
An integrated optical waveguide device is a planar layer or a channel formed in a material adapted to a specific application. Some optical waveguide circuits may contain elements familiar from bulk optics such as mirrors, gratings, lenses, and, in suitable materials, electro optic, or acousto-optic, or magneto-optic modulators. In addition to their compatibility with optical fibers, other advantages of optical waveguide devices include permanent alignment of components, compatibility with existing planar processing technology, diffraction-free beam propagation, low voltage and/or power requirements, high modulation rates, and freedom from electromagnetic interference.
One popular waveguide device is the integrated optic Mach-Zehnder interferometer. The structure of a Mach-Zehnder interferometer consists of an input Y-branch, an output Y-branch, and two parallel waveguide channels that connect the two Y-branches.
An integrated optic Mach-Zehnder interferometer typically is fabricated by proton exchange or titanium diffusion into an electro-optic crystal material, such as lithium niobate, to form optical waveguide channels. The waveguide channels thus created have a higher refractive index region compared to the surrounding substrate material. Hence, light input into the channel is confined to propagate within the channel. Lithium niobate, LiNbO.sub.3, has been one of the most extensively studied materials for integrated optic applications, primarily because of properties that lend themselves to simplicity of low-loss guided wave circuit fabrication and high electro-optic efficiency.
The integrated optic Mach-Zehnder interferometer consists of an input channel that propagates the light to the input Y-branch. At the input Y-branch, the light is equally split between the two channels of the interferometer. For a perfectly symmetric junction, the optical signal will be divided equally between the two legs of the interferometer. The two channels branch out at an angle of, for example, one degree, to a separation of, for example, about 50 microns. The two channels are bent so that they are parallel to each other over a length, L. The two parallel channels are then recombined at the output Y branch and the recombined light is transmitted into the output port. If the two inputs are equal and in phase, the output of the Y-branch will be equal to the sum of the two input intensities. But, if the amplitudes of the two inputs are equal but opposite in phase, the output of the Y-branch will be zero. The energy that does not reach the output leg is radiated into the substrate. In principle, the Y-branch is a four-port device; the three legs of the Y-branch account for three ports and the substrate is the fourth port.
The lengths of the two legs of the Mach-Zehnder interferometer are often designed to be unequal so that the recombining light beams are in phase quadrature with each other. This design condition permits operating the device in its linear region to yield optimum performance and the highest dynamic range.
By positioning appropriate electrodes on or near the parallel waveguide channels, this device can sense electric field. Thus, in a conventional electric field sensor, an external field to be measured is picked up by an antenna and converted to a voltage which is then applied to the electrodes on the lithium niobate crystal (FIG. 1).
Conventionally, electrodes are positioned on top of each waveguide channel in the crystal substrate to create electric fields in opposite directions in each of the two legs of the interferometer. The electrodes are connected so that the two parallel channels experience opposite polarities relative to each other. In one channel, the electric field vector is pointing in one direction, whereas in the other channel, the electric field vector is pointing in a direction opposite to that of the first field vector, thereby leading to a net phase shift of twice the phase shift per channel. The resulting change in the index of refraction of the crystal delays the light in one channel with respect to the light in the adjacent channel, resulting in a net phase shift. As a result, the interferometer operates in a push-pull fashion. When the light is recombined, a change in light intensity results due to the interference of the two light waves.
The interferometer can be operated such that the light intensity is proportional to the electric field across the crystal. However, the conventionally required electrodes and antennas perturb the electric field being measured and create frequency limitations due to their interactions with the electromagnetic fields.
In a variety of applications that require the measurement of electric fields, the presence of a metal electrode tends to disturb the electric field under measurement. In severe cases, the close proximity of the electrodes could create arcing, thus creating a short circuit. The metal electrode also tends to limit the frequency response of the sensor due to the capacitive nature of the electrical circuit. This electrical circuit could also pose a safety hazard in the presence of combustible or explosive materials.
For a Mach-Zehnder interferometer without any electrodes, when the device is placed in a uniform external field, both the channels experience the electric field vector in the same direction. As a result, the phase shift in the two channels is identical, and the net phase shift is zero. If one of the waveguide channels could have its ferroelectric domains reversed, then an external vertically directed electric field would produce equal and opposite phase shifts in the two channels, leading to a total phase shift as experienced by the conventional electroded device. Thus by selectively reverse poling one leg of a Mach-Zehnder interferometer, a non-metallic electric field sensor device could be fabricated.